1. Field of the Invention
This invention relates to the preparation of alkali metal polysilicate solutions, particularly alkali metal polysilicate solutions having a high silica to alkali metal oxide content, say, a silica to alkali metal oxide mole ratio of 2.5 to 5.5:1. This invention is particularly directed to the preparation of lithium polysilicate solutions and especially lithium polysilicate solutions having a high silica to lithium oxide mole ratio. This invention is particularly directed to a simple and relatively commercially feasible process for the preparation of these high silica containing alkali metal polysilicate solutions.
2. Discussion of the Prior Art
Sodium and potassium polysilicate solutions, also known as water glasses, are obtained by melting quartz sand with soda or potash and then dissolving the melt with water under pressure. In this process not only the high expenditure of energy and the high cost of the apparatus are disadvantageous, but also the fact that solutions are obtained in which the maximum molar ratio of SiO.sub.2 to Na.sub.2 O or SiO.sub.2 to K.sub.2 O can be no more than about 3.9 to 1. In the applications of polysilicate solutions, however, molar ratios are desired ranging between 4:1 and 5:1, and therefore this prior art process is not suitable for the preparation of particularly desirable polysilicate solutions.
Lithium polysilicate solutions having this molar ratio and a lower one cannot, however, be prepared in the manner described above, because in this process it is mainly metasilicate and disilicate that form, which are poorly soluble in water. Lithium polysilicate is therefore generally prepared by reacting an amorphous SiO.sub.2 with aqueous LiOH solution. Only if special reaction conditions are precisely maintained does this reaction result in a polysilicate solution. By-products which form very easily are disilicates and metasilicates or even insoluble hydrous silicates of various composition which are insoluble in the reaction mixture. These lithium silicates must therefore be filtered out of the solution in a separate procedure, which often entails complications on account of the high alkalinity and the high viscosity of the solutions.
Several methods have been proposed for circumventing these complications, such as for example operating at elevated temperature with the application of pressure, or with the maintenance of certain stirring speeds, or the hot filtration of the polysilicates which are insoluble at elevated temperatures and dissolve again upon the cooling of the precipitate. These procedures, however, require additional investment in apparatus and do not solve the problem of the additional production of lithium silicates which are insoluble in water, so that a refining filtration is always additionally required as the final procedure.
In these above-named processes, another disadvantage is that one must start out with an amorphous, porous SiO.sub.2 which first has to be produced in a separate process involving a number of steps. An amorphous but solid SiO.sub.2, such as is present, for example, in quartz glass, is unsuitable for this process.
Lithium polysilicate solutions may be prepared from SiO.sub.2 sols by adding a solution of LiOH to them. Two methods of procedure are possible: either an SiO.sub.2 sol stabilized with Na.sup.+ ions or K.sup.+ ions is reacted with the LiOH solution to produce a lithium polysilicate contaminated by these alkali ions, or an alkali-ion-free SiO.sub.2 sol is used which is obtained by passing an SiO.sub.2 sol containing alkali ions through an ion exchanger. Both procedures have disadvantages: either an impure lithium polysilicate is obtained, or an additional investment in apparatus is required on the basis of the treatment with the ion exchanger.
It is, therefore, an object of the present invention to provide a simple and inexpensive process for the preparation of sodium and potassium polysilicate solutions, especially those solutions having a silica to sodium oxide or potassium oxide mole ratio greater than 3.9:1. Another object of the present invention is to provide a process for the preparation of lithium polysilicate solutions wherein the lithium polysilicate solutions are substantially free of extraneous alkali metal ions such as sodium ions and potassium ions. It is another object of the present invention to provide a process for the preparation of such polysilicate solutions which process can be performed in the absence of elevated pressure. It is still a further object of the present invention to provide a process for the preparation of alkali metal polysilicate solutions using as a source of silica and oxygen a tetraalkoxysilane.